The present invention relates to servo actuated valves and particularly to valves of the type employing a pivoted vane or butterfly plate and specifically to butterfly valves employed for controlling flow of water to a heat exchanger for a vehicle passenger compartment heating system. Heretofore, it has been common practice to employ a vacuum servo motor for opening and closing a butterfly water valve for usage with automotive passenger compartment heaters. Typically in such automotive applications the user operated a vacuum valve remote from the water valve to control vacuum applied to one side of a pressure responsive diaphragm actuator mounted on the water valve.
However, it has been desired in modern automotive heater control applications to provide for electrical control of the heater water valve in order to enable interfacing of the heater controls with the controls for the air conditioning system and other vehicular body accessories controlled by an on-board microcomputer. Where an electric motor driven servo actuator has been used for butterfly-type water valves, it has been found difficult to accurately sense the fully open position in order to cut off power to the motor at the desired limit position of the valve movement. One technique which has been employed is that of sensing motor stall current and switching off current to the motor when a predetermined level of current is sensed.
In automotive applications, the available vehicle on-board power is usually that of a 12 volt direct current source. Providing motor driven actuators for accessories operating on a 12 volt DC supply, has required that the motors be of a generally high rpm low current and torque variety in order to operate efficiently. Hence, it has been found necessary to employ a substantial speed reduction in the form of a high numerical ratio gear train to provide the necessary slow rate of rotation and high torque required to open and close the valve.
Where a high rpm electric motor has been employed with a numerically high ratio gear reduction for opening and closing a valve in an automotive heater application, it has been found that upon the valve reaching a limit stop for the open position, the rate of sudden deceleration of the high rpm motor and associated gearing creates torque spikes of high level and short duration associated with the inertial energy of the motor and gears. These torque spikes can damage and/or seize the motor actuator preventing proper operation.
Therefore, it has long been desired to find a way or means of providing an electric motor driven servo actuator operating on low voltage supply such as found in automotive application which could operate between travel limits resulting in motor stall and yet permit such operation without resultant damage to the motor drive.